In battery protecting applications, typically, two Metal Oxide Semiconductor Field Effect Transistors (MOSFETS) are used to control charging and discharging loops. One MOSFET is employed to turn on and turn off the discharging loop and the other MOSFET is employed to turn on and turn off the charging loop. It should be noted that the MOSFETs have inherent body diodes as shown in Prior Art FIGS. 1 and 2.
Referring to PRIOR ART FIG. 1, a series connection topology 100 for charging and discharging a battery 110 in the prior art is illustrated. The series connection topology 100 has two MOSFETs 104 and 102 which are coupled in series. The MOSFET 104 has a body diode 114, and is used to enable or disenable the charging loop. The MOSFET 102 has a body diode 112, and is used to enable or disenable the discharging loop. A load or a power source 130 is coupled to nodes 140 and 142.
The MOSFET 102 and the MOSFET 104 have to meet certain standards, such as high current capacity and low on-state resistance (Ron), since they carry the same current. In order for the MOSFETs 102 and 104 to meet these standards, the cost of the series connection topology 100 is prohibitively high.
In addition, while the MOSFET 104 is turned on to enable the charging loop, the MOSFET 102 will also be turned on to reduce the voltage loss. Similarly, for the discharging loop, both the MOSFETs 102 and 104 are turned on. As such, the MOSFETs 102 and 104 are always turned on, so the insert impedance is (Ron2+Ron3), where the Ron2 is the on-state resistance of the MOSFET 102, and the Ron3 is the on-state resistance of the MOSFET 104. Therefore, the impedance is comparatively high, which means more power loss.
Referring to PRIOR ART FIG. 2, another connection topology 200 in the prior art, namely a parallel connection, is illustrated. In a high power application, this type of topology is preferable.
The parallel connection topology 200 has a MOSFET 204 and a MOSFET 202 which are coupled in parallel. The MOSFET 204 has a body diode 214, and is used to enable or disenable the charging loop. The MOSFET 202 has a body diode 212, and is used to enable or disenable the discharging loop. The charging loop is independent of the discharging loop.
For the discharging loop, a load 222 is coupled to nodes 240 and 242. The discharging current flows through the load 222 and the MOSFET 202. The insert impedance is only the on-state resistance of the MOSFET 202 (Ron2). Accordingly, the discharging loop has a low power loss. Similarly, for the charging loop, an external power source 220 is coupled to nodes 240 and 244, and the charging current flows through the MOSFET 204. The insert impedance is only the on-state resistance of the MOSFET 204 (Ron3). It should be noted that the charging and discharging currents flow through the MOSFET 204 and the MOSFET 202, respectively. Therefore, the MOSFETs 204 and 202 are able to use different types of MOSFETs. For example, since, the power is provided by the external power source 220 and is not significant during charging, the MOSFET 204 in the topology 200 is able to have a high on-state resistance to save cost.
However, the power source 220 may be in failure. For example, in the topology 200 shown in FIG. 2, the nodes 240 and 244 serving as output ports may be shorted, shown as a circuit 201 in PRIOR ART FIG. 3a, or the nodes 240 and 244 are plugged in reverse, shown as a circuit 203 in PRIOR ART FIG. 3b. In either case, the battery 210 will be discharged through the body diode 214 in the MOSFET 204, even if the MOSFET 204 is turned off. That is, the charging loop cannot be cut off completely in both of these cases illustrated in PRIOR ART FIGS. 3a and 3b. Moreover, because the discharging loop and the charging loop are independent from each other in FIG. 2, the charging and discharging current cannot be sensed by one feedback signal, which increases the cost.